This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Our collaborators have previously isolated from tissue extracts a distinct and abundant mammalian Pol II subpopulation that contains an additional tightly associated polypeptide, Gdown1. Their results established that Gdown1-containing Pol II, designated Pol II(G), is selectively dependent on and responsive to Mediator. We are currently carrying out a thorough mass spectrometric analysis of proteins that interact with Gdown1